Transcript Document

Enhanced Da H-mode on Alcator C-Mod
presented by J A Snipes
with major contributions from
M Greenwald, A E Hubbard, D Mossessian,
and the Alcator C-Mod Group
MIT Plasma Science and Fusion Center
Cambridge, MA 02139 USA
Seminar IPP Garching
Garching, Germany
7 May 2002
Global Features of EDA H-Mode
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EDA H-modes have:
 Good energy confinement
H89 ~ 2
 Low particle confinement
no impurity accumulation
 Low radiated power
 No large ELMs
 Steady State (>8tE)
Obtained with Ohmic or ICRF
heating, 1 < PRF< 5 MW
Highly attractive reactor
regime (no ELM erosion)
Similar to LPCH-mode (JET)
and type II ELM regimes
A. Hubbard
Temperature and Density Profiles in
EDA H-mode
• Steep edge temperature and density gradients
• Moderately peaked temperature profile
• Flat density profile
Quasi-Coherent Signature of EDA H-mode
 Enhanced Da emission in EDA H-mode
 f ~100 kHz Quasi-Coherent density and
magnetic fluctuations always found in
EDA H-mode in the steep gradient edge
 QC mode well correlated with reduced
particle and impurity confinement
No large Type I ELMs found on C-Mod
Only small irregular ELMs sometimes
found on top of the enhanced Da emission
M. Greenwald
Edge Pedestal and Fluctuation
Diagnostics
A. Hubbard
Quasi-Coherent Mode seen in Density
Fluctuations in EDA H-modes
• Quasi-coherent edge mode
always associated with EDA
H-Mode
• After brief ELM-free period
(~20 msec), mode appears
• Frequency in lab frame
decreases after onset (
~100 kHz in steady state)
– change in poloidal rotation
• Reflectometer localizes mode
to density pedestal
Y. Lin
Phase Contrast Imaging measures
kR ~ 6 cm-1 (l~1 cm)
• Frequency range 60-250 kHz
• Width DF/F ~ 0.05-0.2
A. Mazurenko
•PCI measures k radially at top
and bottom of plasma.
k R ~ 2 k for typical equilibria
 k s
0.1
Steady Edge Pedestals in EDA
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EDA pedestal characterized by
steep pressure gradients
Pedestal parameters obtained
from tanh fit to measured
Thomson scattering profiles
Moderate pedestal Te (< 500 eV)
and high collisionality n* > 2
Steady-state conditions
throughout ICRF pulse
Quasicoherent mode observed by
reflectometer channel that views
plasma region near the middle of
the pedestal
D. Mossessian
Conditions Favoring EDA
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EDA formation favored by:
– Moderate safety factor
• q95 > 3.5 in D
• q95 > 2.5 (or lower) in H
– Stronger shaping
• d > 0.35
– Higher L-mode target density
• ne > 1.21020 m-3
– Clean wall conditions
(boronization)
Seen in both Ohmic and ICRF
heated discharges
Seen with both favorable and
unfavorable drift direction.
M. Greenwald
Higher density at L-H favours EDA
Low density, ELM-free
Higher density, EDA
Da
Da
ne
ne
•Actual threshold may be in neutral density, local ne or gradient or
collisionality (all are correlated; n*ped < 1 at low ne, 5-10 at high ne)
• 1.21020 m-3 quite low for C-mod. ~0.15 nGW , low ne limit ~0.9 1020
A. Mazurenko
EDA/ELM-free Operational Boundaries
EDA favors high q95 > 3.5 1
and moderate edge
150 < Teped < 500 eV
ELM-free plasmas are more
likely at low q95 and at lower
densities and hence higher
edge temperatures
0.6 MA < Ip < 1.3 MA
4.5 T < Bt < 6 T
1 MW < PRF < 5 MW
D. Mossessian
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M. Greenwald, Phys. Plasmas 6, 1943 (1999)
EDA/ELM-free Operational Boundaries
EDA favors high q95 > 3.5 1
and high edge collisionality
n*ped > 2
ELMy H modes occupy the
same q-n* region as EDA
ELM-free plasmas are more
likely at low q95 and at lower
collisionality
Collisionality n*ped
calculated on 95% yn (top of
the pedestal)
D. Mossessian
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M. Greenwald, Phys. Plasmas 6, 1943 (1999)
Edge Gradients Challenge MHD Limit
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Edge electron profiles from high
resolution Thomson scattering
– assume Ti = Te
Modeling shows gradients are
~30% above the first stability
ballooning limit with only ohmic
current.
– Edge bootstrap current
increases stability limit
No Type I ELMs
(PRF5 MW, P12 MPa/m)
– Small ELMs when bN1.2
D. Mossessian
EDA Pedestal Pressure Increases with Ip
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Thomson pedestal electron
pressure gradient in EDA
increases strongly with plasma
current
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Dashed curves are
0.4
pe  2.8I 1.7
P
p sol
J. Hughes
Time evolution of Te, ne pedestals studied
using power ramps
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RF input power continuously
variable, ramped slowly up and
down.
Te, ne measured with ms time
resolution by ECE,
bremsstrahlung array.
Strong hysteresis in net P.
H-mode threshold in Tedge is
found.
Te pedestal varies in height and
width with P
ne pedestal independent of P
(above LH threshold).
A. Hubbard
Small ELMs appear at high input power
Small, bipolar ELMs in Da
at ~ 600 Hz
Plasma exhaust visible on
divertor probe saturation current
ELMs observed in fast magnetic
coil signal
D. Mossessian
QCM exists at moderate Pped and Teped
ELMy
EDA
When Teped 400 eV broadband low
frequency fluctuations observed in
the pedestal region
QC mode reappears when edge is
cooled
ELMs replace the QC mode at high
pedestal Te
D. Mossessian
EDA/ELM-free Boundary in Pped vs Teped
QCM is not observed when
Te >450 eV
ELMy regime exists in high Te,
high Pped region
D. Mossessian
Probe Measurements Confirm Mode Drives
Particle Transport
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Langmuir probes see mode
when inserted into pedestal
(only possible in low power,
ohmic, H-modes)
Amplitude up to ~50% in n, E
Multiple probes on single head
yield poloidal k~4-6 cm-1, in
agreement with PCI
– Propagation in electron
diamagnetic direction
Analysis of n E shows that
the mode drives significant
radial particle transport across
the barrier, G~ 1022 /m2 s
Plumes from probe gas puffs
show Er < 0 at mode location.
(Er > 0 at larger radii).

ne
1 mm
G n E
B. LaBombard
Particle Diffusion Increases with
Quasi-Coherent Mode Amplitude
• Particle source calculated
with Lyman-a emission, ne(r),
and Te(r)
• Effective particle diffusion:
DEFF = (Source - dN/dt)/ n
• As QC mode strength increases:
– Deff increases
– X-ray pedestal width (~Dimp)
increases.
M. Greenwald
QCM has a strong magnetic component
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Pickup coil added to fast-scanning
Langmuir probe.
Frequency of magnetic component
is identical to density fluctuations.
B ~ 3  104 T implies mode
current density in the pedestal ~10
A/cm2 (~10% of edge j).
Mode is only observed
within ~ 2 cm of the LCFS
Mode is NOT seen on the wall and
limiter coils that are 5 cm outside
the LCFS (at least 1000x lower)
J. Snipes
Magnetic QCM amplitude decreases
rapidly with radius
• Scanning magnetic probe
nearly reaches the LCFS
• Mode decays as
~
~
B  BLCFS exp(  kr ( r  rLCFS ))
• Local QCM kr~1.5 cm-1
10 cm above the outboard
midplane
• Differs from Type III ELM
precursor kr~0.5 cm-1 seen
on the limiter probes
J. Snipes
QCM Poloidal Mode Structure
 Frequency sweeps from > 200 kHz
to ~ 100 kHz just after L-H transition
 Strong magnetic component only
observed within ~2 cm of LCFS
 kr  k  1.5 cm-1 (l  4 cm) near
the outboard midplane
 Assuming a field aligned
perturbation with k  B  0 , k is
expected to vary with position as
k1 / k 2  ( R2 / R1 )2 ( B 2 / B1 )
consistent with PCI kR ~ 6 cm-1 along
its vertical line of sight near the core
J. Snipes
QCM Toroidal Mode Structure
 QCM is sometimes observed on a
toroidal array of outboard limiter
coils
 When the outer gap  1 cm
 Toroidal mode number
15 < n < 18
 At q95 = 5, for a mode resonant at
the edge this implies
75 < m < 90
which is consistent with
<k> ~ 4 cm-1
J. Snipes
Toroidal mode number
Comparison with other
‘small ELM’ regimes
EDA H-mode shares some characteristics of other steady regimes
without large ELMS.
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Low Particle Confinement regime on JET
– Appears similar to EDA, but not easily reproduced.
Quasi-coherent Fluctuations on PDX
– Fluctuations similar to those in EDA, present in short bursts in most Hmodes. Coexisted with ELMs.
Type II or Grassy ELMs on DIII-D, JT60U, Asdex UG
– Conditions in q, d very similar to EDA
– Similar to small ELMs seen in EDA at high bN?
– Does a quasi-coherent mode play a role in these regimes?
Quiescent H-Mode on DIII-D
– Globally similar, but longer wavelength mode, different access
conditions (esp density/neutrals).
A. Hubbard
LPCH-mode on JET Similar to EDA
EDA H-mode in C-Mod
LPCH-mode in JET
J. Snipes
Bout Simulations of the QCM
X.Q. Xu, W.M. Nevins, LLNL
BOUT simulations find an X-point resistive ballooning mode that
is driven in the edge steep gradient region
has a similar magnetic perturbation amplitude and radial structure as the QCM
has a similar dominant k ~ 1.2 cm-1 at the outboard midplane as the QCM
Physical origin of EDA, fluctuations
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Since pedestal profiles are not much different in EDA, ELM-free H-modes,
it seems likely to be the mode stability criteria which change with q,d, n*
etc.
One possibility is that EDA is related to drift ballooning turbulence.
Diamagnetic stabilization threshold scales as m1/2/q.
A lower q threshold was found for EDA in H than D.
Initial scalings of QC mode characteristics show
n  s k  0.1  0.2
 s
n Dn
Electromagnetic edge turbulence simulations by Rogers et al have shown a
feature similar to QC mode, with k 2 / D p .
Gyrokinetic simulations of growth rates (GS2 code) are in progress.
M. Greenwald
Summary
• EDA H-mode combines good energy confinement and
moderate particle confinement in steady state, without large
ELMs
• Edge pedestals have few mm widths, gradients above first
stable limit; but stable with bootstrap currents
• Quasicoherent pedestal fluctuations QCM in density, potential
and B are a key feature of EDA and only occur when:
n*ped > 2, Pped < 1.2x106 Pa/(Wb/rad), Teped <450 eV
• At higher Pped, high Teped QC mode is replaced by small
grassy ELMs
• The observed fluctuations drive significant particle flux
• QCM’s are tentatively identified as resistive ballooning modes